1). Field of the Invention
This invention relates to a method of making a microelectronic assembly.
2). Discussion of Related Art
Integrated circuits are usually manufactured in and on semiconductor substrates that are subsequently “diced” or “singulated” into individual microelectronic dies. Interconnection elements are often formed on a surface of each microelectronic die before the microelectronic dies are singulated.
The interconnection elements are then placed on substrate terminals of a carrier substrate. The entire assembly is then usually placed in a reflow oven which melts the interconnection elements. Subsequent cooling of the interconnection elements causes attachment of the interconnection elements to the substrate terminals. The interconnection elements are thus soldered to the substrate terminals. A solder flux is usually provided to remove metal oxides from the interconnection elements while being soldered. The solder flux is subsequently washed out.
When an integrated circuit in such a die is operated, the integrated circuit generates heat which spreads to the remainder of the microelectronic die and to the substrate. The microelectronic die is usually made of silicon and the substrate of another material, typically an organic polymer material. Differences in coefficients of thermal expansion cause differences in expansion rates of the microelectronic die and the substrate when the heat is generated by operating the circuit, or when the assembly is manufactured. The relative expansion between the microelectronic die and the substrate creates stresses that are especially large at interfaces between the interconnection elements and the substrate terminals.
An underfill material is often provided around the microelectronic die which flows into a space between the microelectronic die and the substrate under capillary action. The underfill material is then heated to a temperature and for a period of time sufficient to cure the underfill material. Curing of the underfill material hardens the underfill material. The hardened underfill material can distribute stresses due to differences in coefficients of thermal expansion, and so prevent the interconnection elements from shearing off the substrate terminals.
No-flow underfill materials are often provided to replace both the solder flux and the conventional underfill material. A no-flow underfill material can remove metal oxides when the interconnection elements are soldered and can subsequently be cured and hardened.
No-flow underfill materials typically do not have the good wetting and flow characteristics of conventional capillary underfill materials. The substrate is usually heated before the no-flow underfill material is dispensed thereon. The substrate then, in turn, heats the underfill material, which improves the wetting characteristics of the underfill material.